You could always say that you learned a lot from them.
It could be true. I had a professor that taught me nothing, but I did learn a lot about how NOT to do things. Like how NOT to act like that in front of people, how NOT to only drone on and on about his research (the epididymus of the rat). How NOT to be an arrogant b@$*@&0 in front of people... I did learn a lot.
dsoz